Update pin_user_pages() calls for Direct I/O

Originally #16856 updated Linux Direct I/O requests to use the new
pin_user_pages API. However, it was an oversight that this PR only
handled iov_iter's of type ITER_IOVEC and ITER_UBUF. Other iov_iter
types may try and use the pin_user_pages API if it is available. This
can lead to panics as the iov_iter is not being iterated over correctly
in zfs_uio_pin_user_pages().

Unfortunately, generic iov_iter API's that call pin_user_page_fast() are
protected as GPL only. Rather than update zfs_uio_pin_user_pages() to
account for all iov_iter types, we can simply just call
zfs_uio_get_dio_page_iov_iter() if the iov_iter type is not ITER_IOVEC
or ITER_UBUF. zfs_uio_get_dio_page_iov_iter() calls the
iov_iter_get_pages() calls that can handle any iov_iter type.

In the future it might be worth using the exposed iov_iter iterator
functions that are included in the header iov_iter.h since v6.7. These
functions allow for any iov_iter type to be iterated over and advanced
while applying a step function during iteration. This could possibly be
leveraged in zfs_uio_pin_user_pages().

A new ZFS test case was added to test that a ITER_BVEC is handled
correctly using this new code path. This test case was provided though
issue #16956.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes #16956 
Closes #17006

config/kernel-vfs-iov_iter.m4

@@ -21,6 +21,20 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_VFS_IOV_ITER], [
 		__attribute__((unused)) enum iter_type i = iov_iter_type(&iter);
 	])
 
+	ZFS_LINUX_TEST_SRC([iov_iter_get_pages2], [
+		#include <linux/uio.h>
+	],[
+		struct iov_iter iter = { 0 };
+		struct page **pages = NULL;
+		size_t maxsize = 4096;
+		unsigned maxpages = 1;
+		size_t start;
+		size_t ret __attribute__ ((unused));
+
+		ret = iov_iter_get_pages2(&iter, pages, maxsize, maxpages,
+		    &start);
+	])
+
 	ZFS_LINUX_TEST_SRC([iter_is_ubuf], [
 		#include <linux/uio.h>
 	],[
@@ -64,6 +78,19 @@ AC_DEFUN([ZFS_AC_KERNEL_VFS_IOV_ITER], [
 		AC_MSG_RESULT(no)
 	])
 
+
+	dnl #
+	dnl # Kernel 6.0 changed iov_iter_get_pages() to iov_iter_get_pages2().
+	dnl #
+	AC_MSG_CHECKING([whether iov_iter_get_pages2() is available])
+	ZFS_LINUX_TEST_RESULT([iov_iter_get_pages2], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_IOV_ITER_GET_PAGES2, 1,
+		    [iov_iter_get_pages2() is available])
+	],[
+		AC_MSG_RESULT(no)
+	])
+
 	dnl #
 	dnl # Kernel 6.0 introduced the ITER_UBUF iov_iter type. iter_is_ubuf()
 	dnl # was also added to determine if the iov_iter is an ITER_UBUF.

include/os/linux/spl/sys/uio.h

@@ -63,6 +63,7 @@ typedef enum zfs_uio_seg {
 typedef struct {
 	struct page	**pages;	/* Mapped pages */
 	long 		npages;		/* Number of mapped pages */
+	boolean_t	pinned;		/* Whether FOLL_PIN was used */
 } zfs_uio_dio_t;
 
 typedef struct zfs_uio {
@@ -199,4 +200,13 @@ zfs_uio_iov_iter_init(zfs_uio_t *uio, struct iov_iter *iter, offset_t offset,
 #define	zfs_uio_iov_iter_type(iter)	(iter)->type
 #endif
 
+#if defined(HAVE_ITER_IS_UBUF)
+#define	zfs_user_backed_iov_iter(iter)	\
+	(iter_is_ubuf((iter)) || \
+	(zfs_uio_iov_iter_type((iter)) == ITER_IOVEC))
+#else
+#define	zfs_user_backed_iov_iter(iter) \
+	(zfs_uio_iov_iter_type((iter)) == ITER_IOVEC)
+#endif
+
 #endif /* SPL_UIO_H */

module/os/linux/zfs/zfs_uio.c

@@ -404,7 +404,6 @@ zfs_uio_page_aligned(zfs_uio_t *uio)
 	return (aligned);
 }
 
-
 #if defined(HAVE_ZERO_PAGE_GPL_ONLY) || !defined(_LP64)
 #define	ZFS_MARKEED_PAGE	0x0
 #define	IS_ZFS_MARKED_PAGE(_p)	0
@@ -441,7 +440,6 @@ zfs_unmark_page(struct page *page)
 }
 #endif /* HAVE_ZERO_PAGE_GPL_ONLY || !_LP64 */
 
-#if !defined(HAVE_PIN_USER_PAGES_UNLOCKED)
 static void
 zfs_uio_dio_check_for_zero_page(zfs_uio_t *uio)
 {
@@ -473,7 +471,6 @@ zfs_uio_dio_check_for_zero_page(zfs_uio_t *uio)
 		}
 	}
 }
-#endif
 
 void
 zfs_uio_free_dio_pages(zfs_uio_t *uio, zfs_uio_rw_t rw)
@@ -482,21 +479,24 @@ zfs_uio_free_dio_pages(zfs_uio_t *uio, zfs_uio_rw_t rw)
 	ASSERT(uio->uio_extflg & UIO_DIRECT);
 	ASSERT3P(uio->uio_dio.pages, !=, NULL);
 
+	if (uio->uio_dio.pinned) {
 #if defined(HAVE_PIN_USER_PAGES_UNLOCKED)
-	unpin_user_pages(uio->uio_dio.pages, uio->uio_dio.npages);
-#else
-	for (long i = 0; i < uio->uio_dio.npages; i++) {
-		struct page *p = uio->uio_dio.pages[i];
-
-		if (IS_ZFS_MARKED_PAGE(p)) {
-			zfs_unmark_page(p);
-			__free_page(p);
-			continue;
-		}
-
-		put_page(p);
-	}
+		unpin_user_pages(uio->uio_dio.pages, uio->uio_dio.npages);
 #endif
+	} else {
+		for (long i = 0; i < uio->uio_dio.npages; i++) {
+			struct page *p = uio->uio_dio.pages[i];
+
+			if (IS_ZFS_MARKED_PAGE(p)) {
+				zfs_unmark_page(p);
+				__free_page(p);
+				continue;
+			}
+
+			put_page(p);
+		}
+	}
+
 	vmem_free(uio->uio_dio.pages,
 	    uio->uio_dio.npages * sizeof (struct page *));
 }
@@ -523,6 +523,7 @@ zfs_uio_pin_user_pages(zfs_uio_t *uio, zfs_uio_rw_t rw)
 	if (len == 0)
 		return (0);
 
+	uio->uio_dio.pinned = B_TRUE;
 #if defined(HAVE_ITER_IS_UBUF)
 	if (iter_is_ubuf(uio->uio_iter)) {
 		nr_pages = DIV_ROUND_UP(len, PAGE_SIZE);
@@ -569,8 +570,8 @@ zfs_uio_pin_user_pages(zfs_uio_t *uio, zfs_uio_rw_t rw)
 
 	return (0);
 }
+#endif
 
-#else
 static int
 zfs_uio_get_dio_pages_iov_iter(zfs_uio_t *uio, zfs_uio_rw_t rw)
 {
@@ -581,9 +582,15 @@ zfs_uio_get_dio_pages_iov_iter(zfs_uio_t *uio, zfs_uio_rw_t rw)
 	unsigned maxpages = DIV_ROUND_UP(wanted, PAGE_SIZE);
 
 	while (wanted) {
+#if defined(HAVE_IOV_ITER_GET_PAGES2)
+		cnt = iov_iter_get_pages2(uio->uio_iter,
+		    &uio->uio_dio.pages[uio->uio_dio.npages],
+		    wanted, maxpages, &start);
+#else
 		cnt = iov_iter_get_pages(uio->uio_iter,
 		    &uio->uio_dio.pages[uio->uio_dio.npages],
 		    wanted, maxpages, &start);
+#endif
 		if (cnt < 0) {
 			iov_iter_revert(uio->uio_iter, rollback);
 			return (SET_ERROR(-cnt));
@@ -595,7 +602,12 @@ zfs_uio_get_dio_pages_iov_iter(zfs_uio_t *uio, zfs_uio_rw_t rw)
 		uio->uio_dio.npages += DIV_ROUND_UP(cnt, PAGE_SIZE);
 		rollback += cnt;
 		wanted -= cnt;
+#if !defined(HAVE_IOV_ITER_GET_PAGES2)
+		/*
+		 * iov_iter_get_pages2() advances the iov_iter on success.
+		 */
 		iov_iter_advance(uio->uio_iter, cnt);
+#endif
 
 	}
 	ASSERT3U(rollback, ==, uio->uio_resid - uio->uio_skip);
@@ -603,7 +615,6 @@ zfs_uio_get_dio_pages_iov_iter(zfs_uio_t *uio, zfs_uio_rw_t rw)
 
 	return (0);
 }
-#endif /* HAVE_PIN_USER_PAGES_UNLOCKED */
 
 /*
  * This function pins user pages. In the event that the user pages were not
@@ -621,7 +632,10 @@ zfs_uio_get_dio_pages_alloc(zfs_uio_t *uio, zfs_uio_rw_t rw)
 	if (uio->uio_segflg == UIO_ITER) {
 		uio->uio_dio.pages = vmem_alloc(size, KM_SLEEP);
 #if defined(HAVE_PIN_USER_PAGES_UNLOCKED)
-		error = zfs_uio_pin_user_pages(uio, rw);
+		if (zfs_user_backed_iov_iter(uio->uio_iter))
+			error = zfs_uio_pin_user_pages(uio, rw);
+		else
+			error = zfs_uio_get_dio_pages_iov_iter(uio, rw);
 #else
 		error = zfs_uio_get_dio_pages_iov_iter(uio, rw);
 #endif
@@ -632,22 +646,24 @@ zfs_uio_get_dio_pages_alloc(zfs_uio_t *uio, zfs_uio_rw_t rw)
 	ASSERT3S(uio->uio_dio.npages, >=, 0);
 
 	if (error) {
+		if (uio->uio_dio.pinned) {
 #if defined(HAVE_PIN_USER_PAGES_UNLOCKED)
-		unpin_user_pages(uio->uio_dio.pages, uio->uio_dio.npages);
-#else
-		for (long i = 0; i < uio->uio_dio.npages; i++)
-			put_page(uio->uio_dio.pages[i]);
+			unpin_user_pages(uio->uio_dio.pages,
+			    uio->uio_dio.npages);
 #endif
+		} else {
+			for (long i = 0; i < uio->uio_dio.npages; i++)
+				put_page(uio->uio_dio.pages[i]);
+		}
+
 		vmem_free(uio->uio_dio.pages, size);
 		return (error);
 	} else {
 		ASSERT3S(uio->uio_dio.npages, ==, npages);
 	}
 
-#if !defined(HAVE_PIN_USER_PAGES_UNLOCKED)
-	if (rw == UIO_WRITE)
+	if (rw == UIO_WRITE && !uio->uio_dio.pinned)
 		zfs_uio_dio_check_for_zero_page(uio);
-#endif
 
 	uio->uio_extflg |= UIO_DIRECT;
 

tests/runfiles/linux.run

@@ -103,7 +103,7 @@ tests = ['devices_001_pos', 'devices_002_neg', 'devices_003_pos']
 tags = ['functional', 'devices']
 
 [tests/functional/direct:Linux]
-tests = ['dio_write_verify']
+tests = ['dio_loopback_dev', 'dio_write_verify']
 tags = ['functional', 'direct']
 
 [tests/functional/events:Linux]

tests/zfs-tests/tests/Makefile.am

@@ -1476,6 +1476,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/direct/dio_dedup.ksh \
 	functional/direct/dio_encryption.ksh \
 	functional/direct/dio_grow_block.ksh \
+	functional/direct/dio_loopback_dev.ksh \
 	functional/direct/dio_max_recordsize.ksh \
 	functional/direct/dio_mixed.ksh \
 	functional/direct/dio_mmap.ksh \

tests/zfs-tests/tests/functional/direct/dio_loopback_dev.ksh

@@ -0,0 +1,78 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or https://opensource.org/licenses/CDDL-1.0.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2025 by Triad National Security, LLC.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/direct/dio.cfg
+. $STF_SUITE/tests/functional/direct/dio.kshlib
+
+#
+# DESCRIPTION:
+# 	Verify Direct I/O reads work with loopback devices using direct=always.
+#
+# STRATEGY:
+#	1. Create raidz zpool.
+#	2. Create dataset with the direct dataset property set to always.
+#	3. Create an empty file in dataset and setup loop device on it.
+#	4. Read from loopback device.
+#
+
+verify_runnable "global"
+
+function cleanup
+{
+	if [[ -n $lofidev ]]; then
+		losetup -d $lofidev
+	fi
+	dio_cleanup
+}
+
+log_assert "Verify loopback devices with Direct I/O."
+
+if ! is_linux; then
+	log_unsupported "This is just a check for Linux Direct I/O"
+fi
+
+log_onexit cleanup
+
+# Create zpool
+log_must truncate -s $MINVDEVSIZE $DIO_VDEVS
+log_must create_pool $TESTPOOL1 "raidz" $DIO_VDEVS
+
+# Creating dataset with direct=always
+log_must eval "zfs create -o direct=always $TESTPOOL1/$TESTFS1"
+mntpt=$(get_prop mountpoint $TESTPOOL1/$TESTFS1)
+
+# Getting a loopback device
+lofidev=$(losetup -f)
+
+# Create loopback device
+log_must truncate -s 1M "$mntpt/temp_file"
+log_must losetup $lofidev "$mntpt/temp_file"
+
+# Read from looback device to make sure Direct I/O works with loopback device
+log_must dd if=$lofidev of=/dev/null count=1 bs=4k
+
+log_pass "Verified loopback devices for Direct I/O."